Drive units for a continuously variable rotational speed control are known from EP 0 902 213 A1. This known drive unit is based on the principle: of splitting power, in which the power inputted by way of a common drive shaft, is split into a mechanical power portion in the form of a mechanical transmission unit as well as to a continuously variably controllable power portion in the form of a hydrostat. The mechanical transmission unit has a summing transmission by way of which the continuously variably controllable rotational speed of the hydrostat can be superimposed on the input speed of the mechanical transmission unit. Consequently, the transmission has a motor-driven input and a continuously variably controllable output.
A continuously variable controllable output is also advantageous for crawler-type vehicles where two outputs, one for the left track and one for the right track, are in place and where a reversal of the rotational direction also must be possible, for example, when turning the vehicle in place. Known drive unit for crawler-type vehicles have a conventional transmission and a complex downstream power split transmission, which continuously variably distributes the rotational speed and the direction of rotation to both tracks. The conventional transmission must be designed to be able to handle the full engine torque as well as the full engine power.
From DE-A 2 017 716, a steering gear is known for crawler-type vehicles, in which a first differential gear is superimposed on a second differential gear and in which one and/or the other side of the superimposed differential gear can be fully or partially braked. In this way, the two output shafts, that is, the left and the right track of the crawler-type vehicle, can be driven in opposite rotational directions, but continuously variable rotational speed control is not possible with this type of input.
The engine in vehicles with permanent all-wheel drive must also drive two output shafts. Known all-wheel drives are equipped with a transfer transmission installed downstream of a conventional transmission, which transfers torque and power to the front and rear axles of the vehicle, wherein a locking or non-locking linear differential can also be provided. The conventional transmission and transfer transmission must be designed to be able to handle the full engine torque and the full engine power.
From the Applicant's DE 39 38 888 B4, a transfer transmission for a vehicle is known having an all-wheel drive in which the power originating from the engine is passed through a conventional transmission and is transferred by way of planetary gears to the front and rear axles and both outputs are co-axially arranged. A continuously variable rotational speed control for front and rear axle is not provided for the known transmission.
It is an object of the invention to create a continuously variable rotational speed control with the capability of reversing the rotational direction of two output shafts for a drive unit of the aforementioned type, where the drive unit can also be advantageously used for motor vehicles.